A number of diagnostic procedures are available for evaluating thyroid function. Among the most widely used and accurate of these procedures are the diagnostic tests which measure the binding capacity of the thyroid hormone binding proteins present in a body fluid for the radioisotope labeled thyroid hormones triiodothyronine (T3) and thyroxine (T4). These are commonly referred to in the art as the T3 uptake test in the case of triiodothyronine and the T4 test in the case of thyroxine. In general, the procedures employed by the T3 uptake or T4 diagnostic tests involve the mixing of radioisotope labeled T3 (or T4) to a sample of body fluid, usually serum. Various proteins in the serum have the ability to bind the hormones. A secondary binding agent which can adsorb the hormone not already bound to the serum proteins is mixed with the serum sample. The secondary binding agent with the adsorbed hormone not originally bound to the serum protein can then be separated from the test sample. The radioactivity of either the serum sample or the secondary binding agent is measured in a suitable scintillation counter. Many variations of the general procedure have been developed; most vary in the nature of the secondary binding agent. As used herein, the term body fluid refers to a body fluid such as serum or blood which contains proteins capable of binding the thyroid hormones T3 or T4.
The secondary binding agent used to separate the unbound hormone from the test sample must have physical properties which allow it to be separated from the test solution so that the radioactivity of the separated fractions can be measured. Currently available methods use ion exchange resins in various forms (see U.S. Pat. Nos. 3,206,602; 3,714,344 and 3,414,383) or other adsorbent materials such as talc (see U.S. Pat. No. 3,666,854) and sephadex columns. Rolleri et al., J. Nucl. Med., 13, 893 (1972), describes the use of water-insoluble albumin. Most of the currently available methods have the disadvantage of requiring precise timing during the test procedure. In addition, some of the methods are not convenient for handling of large numbers of tests simultaneously because of the complexity of the procedure.